The present invention relates to ultrasonic measurement apparatus, and in particular, to ultrasonic measurement apparatus for determining the level of a petroleum product stored in an underground tank.
In those industries where large volumes of liquid material are kept in underground storage tanks, it is desirable to be able to easily measure the volume of liquid stored. For example, in the service station industry, current government regulations require that a service station owner record the volume of petroleum products stored underground once every 24 hours. Currently, the method used to determine such volumes comprises a simple dipstick that is inserted into the storage tank. The volume of liquid stored in a tank is computed based on the level of liquid in the tank. The dipstick is calibrated to show the volume of liquid stored in the tank so that the liquid volume can be determined simply by looking at the wet line left on the dipstick as it is raised from the tank. Obviously, such a manual method of volume measurement has its disadvantages. For example, it is difficult to read such a calibrated dipstick in the dark or in the rain. Similarly, intense sunlight can quickly evaporate gasoline, making determination of an accurate wet line position on the dipstick difficult.
There have been numerous attempts to develop an automated volume measuring device; however, such devices have not been proven accurate enough to achieve widespread use in industry. Generally, such devices comprise an ultrasonic transducer and a timing mechanism. The liquid level is determined by the round-trip time it takes an ultrasonic pulse to travel from the transducer to the level of the liquid and back. As with the dipstick method, the volume of liquid stored in the underground tank can be computed if the level of the liquid is known. The problem with such ultrasonic measuring devices is that the level of precision achieved is highly dependent on the velocity of sound in the air above the liquid. Because the velocity of sound in air changes due to the presence of chemical vapor or with changes in temperature, it is necessary to provide a calibration mechanism whereby the speed of sound in the air above the liquid can be compensated.
U.S. Pat. No. 4,210,969, issued to Massa, discloses an ultrasonic liquid level detector that uses a reference reflector located at a precise fixed distance from an ultrasonic transducer. By ratiometrically comparing the time an ultrasonic pulse takes to travel to the reference reflector and back with the time it takes an ultrasonic pulse to travel to the level of a liquid and back, the level of the liquid in the tank can be computed, independent of the velocity of sound in the gaseous medium above the liquid, provided that the velocity of sound remains constant over the traversed distance.
U.S. Pat. No. 4,470,299, issued to Soltz, discloses an ultrasonic level detector that uses a reference reflector placed in a fixed position relative to an ultrasonic transducer to intercept energy from a side signal path and return it to the transducer to produce a reference signal. The reference signal is used in conjunction with a round-trip time it takes an ultrasonic pulse to travel to the liquid level and back to ratiometrically determine the level of liquid in the tank, independent of the velocity of sound in the air above the liquid.
Although the above-mentioned liquid level measuring devices may work well in some environments, they have not proved sufficiently accurate for storage tanks containing liquids having a high vapor pressure. The inventors have discovered that a correction factor is needed for ultrasonic measurements taken in storage tanks that contain petroleum products, to compensate for a non-linear vapor density in the ultrasonic detection path. In storage tanks containing petroleum products such as gasoline or kerosene, the inventors have found that the vapor density changes non-linearly from the top of the fill pipe to the level of the liquid. This changing vapor density causes the velocity of sound to vary accordingly throughout its round trip to the liquid level and back. As a result, it is not possible to accurately ratiometrically determine the level of liquid in a tank containing petroleum products, independent of the velocity of sound, without applying a correction.